1. Field of the Invention
The present invention relates to a high-strength steel machined product having excellent hardenability and a method for manufacturing thereof, and to a method for manufacturing diesel engine fuel injection pipe and common rail having high strength and excellent impact resistance and internal pressure fatigue resistance, specifically to a high-strength steel machined product made of an ultra-high low-alloy TRIP steel (TBF steel) having high hardenability mainly composed of lathy bainitic ferrite, retained austenite, and martensite, exhibiting high yield strength and tensile strength, a high-strength forged product, a high-pressure fuel injection pipe, and a common rail for accumulator fuel injection system mounted on a diesel engine, and to a method for manufacturing thereof.
2. Description of the Related Art
It should be noted that typical examples of the high-strength forged product according to the present invention include near-net shape forged products, encompassing not only primary forged products but also secondary forged products obtained by further forging (such as cold forging and warm forging) the primary forged products, precision forged products such as tertiary forged products, ultimate products obtained by machining these forged products into complex shapes, and a common rail for accumulator fuel injection system mounted on a diesel engine.
Forged products in the industrial fields of automobile, electric equipment, machines, and the like are normally manufactured by performing various forging (machining) methods at different heating temperatures, followed by performing thermal refining (heat treatment) such as hardening and tempering. For example, in an automobile, crank shaft, connecting rod, transmission gear, common rail for accumulator fuel injection system mounted on a diesel engine, and the like normally adopt hot-forged products (pressurizing temperature in a range of 1100° C. to 1300° C.) and warm-forged products (pressurizing temperature in a range of 600° C. to 800° C.), and pinion gear, gear, steering shaft, valve lifter, and the like normally adopt cold-forged products (pressurized at normal temperature).
In recent years, to attain weight reduction of an automobile body and to assure collision safety of automobiles, there have been examined the use of formable ultra-high strength low-alloy TRIP steels (TBF steels) having the transformation-induced plasticity of retained austenite.
For example, Japanese Patent Laid-Open No. 2004-292876 discloses a technology relating to the method for manufacturing high-strength forged product having high elongation and excellent balance of strength and drawing characteristics in a high-strength region giving 600 MPa or larger tensile strength through an exclusive heat treatment of performing austempering at a specified temperature after having performed both annealing and forging generally at a temperature of two-phase region of ferrite and austenite; and Japanese Patent Laid-Open No. 2005-120397 discloses a technology of manufacturing high-strength forged product having high elongation and excellent balance of strength and drawing characteristics by performing both annealing and forging mostly at a temperature of two-phase region of ferrite and austenite and then performing austempering at a specified temperature, after having separately formed tempered bainite or martensite; and Japanese Patent Laid-Open No. 2004-285430 discloses a technology of manufacturing high-strength forged product having excellent stretch flangeability and workability along with allowing the decrease in the temperature at the time of forge processing, by performing forge processing in the two-phase range and then performing specified austempering, after having heated the article to a temperature of two-phase range.
When, however, the forged products obtained by the above disclosed methods are manufactured, problems described below may be raised.
Since a forged product generates heat depending on the processing ratio of the article, the temperature may differ at positions therein during forging. For example, forging at a high temperature (near Ac3 point) increases the generated heat with increase in the processing ratio, and there occurs coalescence and growth of austenite grains, which may induce coarse retained austenite after the heat treatment. Therefore, it can be considered that the impact resistance is deteriorated (problem at the time of high-temperature forging). On the other hand, when forging is performed at a low temperature (near Ac1 point), low processing ratio makes it impossible to secure sufficient generation of heat, which may result in forming a large amount of unstable retained austenite. Thus, it can be considered that the impact resistance is deteriorated because hard martensite is generated as an origin of the fracture after the heat treatment (problem at the time of low-temperature forging). Consequently, when the temperature and processing ratio differ in a forged product, there likely appear coarse retained austenite and unstable austenite in a part, which results in having difficulty in obtaining stable and excellent impact resistance for the entire forged product.
Japanese Patent Laid-Open No. 2007-231353 discloses a technology of manufacturing a steel-made high-strength machined product having excellent impact resistance with high elongation and excellent balance of strength and drawing characteristics giving 600 MPa or larger tensile strength irrespective of the forging temperature and the forging processing ratio, and a high-pressure fuel pipe (specifically diesel engine fuel injection pipe, diesel engine common rail, and the like having high strength and excellent impact resistance) through the addition of one or more of Nb, Ti, and V and an adequate amount of Al at the time of forming a hot-rolled steel, and performing heat treatment of both annealing and forging mostly at a temperature of two-phase range of ferrite and austenite, followed by performing austempering treatment at a specified temperature.
The invention disclosed in Japanese Patent Laid-Open No. 2007-231353 is superior to the technologies disclosed in Japanese Patent Laid-Open No. 2004-292876, Japanese Patent Laid-Open No. 2005-120397 and Japanese Patent Laid-Open No. 2004-285430 at the viewpoint of providing a special effect which cannot be obtained by these technologies, and thus the ultra-high strength low-alloy TRIP steel (TBF steel) manufactured by the invention is expected to significantly contribute to the weight-reduction of automobile bodies and the collision safety of automobiles. Since, however, the ultra-high strength low-alloy TRIP steel (TBF steel) allows the fine grain bainite-ferrite and square-shape ferrite to coexist with the lathy structure of bainite-ferrite in the matrix, there is needed a high hardenability in order to obtain perfect TBF steel for attaining further high yield strength and tensile strength. At present, however, the ultra-high low-alloy TRIP steel (TBF steel) having that high hardenability has not been developed yet.